Continuous process for the recovery of terephthalic acid from waste or used products of polyalkylene terephthalate polymers

Abstract: Continuous process for the recovery of terephthalic acid from waste or used products of polyalkylene terephthalate polymers consisting in subjecting said waste or used products to two consecutive treatments, the first carried out with superheated methanol vapour (methanolysis) and the second with water or water vapour at a high temperature (hydrolysis).

Description: The present invention relates to a process for the recovery of waste and used products of polyalkylene terephthalate. More specifically, the present invention relates to a process for the recovery of terephthalic acid and alkylene glycols from waste and used products of polyalkylene terephthalate. Polyalkylene terephthalates are polymeric products with a high molecular weight which are well-known in the art. They are obtained by the polytransesterification of terephthalic acid or one of its alkyl-esters with an aliphatic glycol. In the specific case of polyethylene terephthalate, which is the most common polyester on the market, this is generally produced by reacting terephthalic acid or one of its alkyl esters, such as dimethyl terephthalate, with ethylene glycol, in the presence of a catalyst, to form the dihydroxyethyl-terephthalate monomer which is subsequently polymerized to a high molecular weight using the known polymerization techniques by condensation. Polyalkylene terephthalates with a high molecular weight, and in particular polyethylene terephthalate, are industrially applied in the production of films for packaging, fibres, electric insulators, moulded articles, food containers, such as bottles, etc. The various operating or production processes generate, however, a considerable quantity of waste or by-products, such as shavings, polymer cuttings, thread and textile waste, etc., which negatively affect the cost of the finished product. In addition, the mass of these products, is disposed after use in rubbish dumps. These waste or used products consequently create enormous problems both for the environment and for their disposal. Various processes have been proposed for recycling these waste products, mainly based on the recovery of their constituents and in particular the most costly component i.e. terephthalic acid. The various known processes have numerous disadvantages however due to the fact that the regenerated products contain impurities which are difficult to separate and which jeopardize their subsequent re-use. One method proposed consists in dissolving the waste products in dihydroxyethyl terephthalate monomer in the molten state and recycling the solution obtained in polycondensation. The polyester thus obtained has lower properties than those of polyester obtained from vergin monomers. Another method proposed consists in degrading the polymer with an alkyl alcohol with a low molecular weight and recycling the degraded product. For example, patent US-A-3.257.335 describes a degradation process of waste products of polyalkylene terephthalate by treatment with ethylene glycol. The process consists in dissolving and reacting the waste products of polyalkylene terephthalate with ethylene glycol to prepare di-hydroxyethyl terephthalate monomer and oligomers with a low molecular weight (dimers, trimers etc.), which are subsequently mixed with the vergin monomer and then polymerized. The polymer obtained with this process has limited properties, especially when a high ratio of recovery products is used. This deterioration of the properties also seems to be due to the formation of diethylene glycol during the reaction with ethylene glycol. As an alternative, patents US-A-2.884.443, US-A-3.488.298 and US-A-3.776.945 propose degradation of the polyalkylene terephthalate with methanol to form dimethyl terephthalate. The dimethyl terephthalate recovered is then added in the transesterification phase in the production process of polyalkylene terephthalate. The main disadvantage of this process is the difficulty of introducing the solid waste products of polyester at the high pressure required to obtain the degradation of the polymer. For this reason the process is carried out in batch with relative problems of storage and all the drawbacks associated with a batch process. A proposal has also been made to carry out the degradation of the waste products of polyalkylene terephthalate with a two-step process. In the first step the waste products are treated with ethylene glycol to prepare a solution containing dihydroxyethylene terephthalate and oligomers with a low molecular weight and in the second step the solution is treated with methanol to further degrade the oligomers and convert the terephthalates to dimethyl terephthalate. A similar process using an exchange catalyst is disclosed in patent DDR-A-68.500. This process however, also has the above disadvantages, as impurities such as diethylene glycol and comonomers and/or additives commonly used in the production of polyester, are entrapped in the dimethyl terephthalate obtained by crystallization of the mother liquor and their elimination by subsequent crystallization is a technically complex and costly operation. European patent EP-A-0.484.963 describes a continuous process for the preparation of dimethyl terephthalate from the waste products of polyalkylene terephthalate by treatment with methanol vapours at a temperature higher than 230°C, continuous removal of methanol vapours, dimethyl terephthalate and olefinic glycol from the reaction zone, separation of the methanol and recovery of the dimethyl terephthalate. Patent US-A-5.015.528 discloses a continuous process for the recovery of ethylene glycol and dimethyl terephthalate from polyethylene terephthalate consisting in passing superheated methanol vapours through a solution obtained by dissolving polymer waste products in an oligomer of ethylene glycol and terephthalic acid or its methyl ester and recovering the ethylene glycol and dimethyl terephthalate by distillation. These continuous processes enable the production of dimethyl terephthalate and ethylene glycol without inorganic residues and heteropolymers, enabling them to be more easily purified but without solving the problem of the separation of the comonomers. These processes, however, have the serious technical problem of having to be carried out extremely close to where the dimethyl terephthalte is used. It is, in fact, known that dimethyl terephthalate is not very stable and there are consequently technical problems for its transport. Processes have also been proposed for the recovery of waste products of polyalkylene terephthalate by both continuous and batch hydrolysis to obtain pure terephthalic acid directly by crystallization. Hydrolysis processes are described in patents US-A-3.321.510, US-A-4.605.762 and GB-A-2.123.403. Not even these processes, however, solve the problem of the impurities due to both the formation of hydroyethyl-methyl terephthalate and to the presence of comonomers of the starting polyester. In addition, the hydrolysis process requires the use of waste products which are absolutely without pigments, heteropolymers or various additives as well as extraneous materials such as paper or aluminium films, and they must consequently by subjected to a preliminary purification process which considerably influences the cost of the products obtained. The present invention provides a continuous process for the recovery of terephthalic acid and alkylene glycol from the waste or used products of the production and transformation of polyester which does not have the above disadvantages. More specifically, the present invention provides a continuous process for the degradation of polyalkylene terephthalate to obtain "fibre grade" terephthalic acid i.e. without impurities which can jeopardize its re-use in the production of polyesters for fibres. In its more general aspect, the process of the present invention consists in subjecting the waste or used products of polyalkylene terephthalate to two consecutive treatments, the first carried out with superheated methanol vapours (methanolysis) and the second with water or water vapour at a high temperature (hydrolysis). The present invention consequently relates to a continuous process for the recovery of terephthalic acid from the waste or used products of polyalkylene terephthalate polymers which comprises in: (a) treating said polymer waste products with methanol vapours, at atmospheric pressure and at a temperature higher than 150°C, to decompose the polymer into its constituent monomers; (b) continuously removing the methanol vapours and monomers obtained in step (a) and entrapped by said methanol vapours; (c) condensing the monomers entrapped by the methanol; (d) separately condensing and recycling the methanol to step (a); (e) removing the glycols from the mixture of condensed monomers by distillation; (f) subjecting the residual product of the distillation to hydrolysis; and (g) recovering the terephthalic acid obtained by crystallization. The process of the present invention enables "fibre grade" terephthalic acid to be obtained starting directly from the waste products, by-products and used products based on or containing polyalkylene terephthalate. The materials subjected to the process of the present invention can be waste products from the production of moulded articles or products such as fibres, films, bottles, etc. as well as the moulded products or articles themselves after use. Low molecular weight oligomers obtained as by-products during the polycondensation or process for the production of the polymer, can also be used. These materials are subjected to treatment with methanol vapours in the form of pellets, filaments or any other form which can be suitably handled. The materials to be treated can also be previously granulated in an extruder or fed to the chamber of a screw extruder which in turn feeds the extruded product directly to the treatment zone. Alternatively, the materials to be treated can be melted and continuously injected to the reaction zone. It is not necessary for the material subjected to the process of the present invention to be free from contaminating products. In fact, the process of the present invention is equally effective even when the polyester fibres to be treated are mixed with other natural, artificial or synthetic fibres; or when the polyester is metallized, dyed, pigmented or mixed with other polymers. Similarly, the presence of extraneous products such as paper, films, packaging products, etc. does not influence the process of the present invention. The treatment with methanol vapours is carried out at atmospheric pressure, at a temperature higher than 150°C, and with an excess of methanol so that the weight ratio methanol/waste or used materials is at least 2, and preferably between 3 and 5. It is preferable for the temperature of the vapours to be higher than the critical temperature of the methanol, i.e. higher than 230°C, in order to avoid liquefying the methanol by increase in pressure; temperatures of between 240 and 260°C are particularly preferred. During this treatment with methanol the polyalkylene terephthalate is depolymerized into its constituent monomers, such as dimethyl terephthalate, monomethyl terephthalate and alkylene glycol, and into oligomeric products having a polymerization degree of between 5 and 20. The monomers are continuously removed from the reactor together with the methanol vapours, whereas the oligomeric products remain in the reactor. The quantity of fed products to be treated and the quantity of monomers removed are regulated so that, under standard conditions, an oligomeric phase with an almost constant composition is present on the bottom of the reactor. The excess methanol fed to the reactor facilitates the removal of the monomers as it acts as transporter. It is preferable for the vapours leaving the reactor to contain at least 50 moles of methanol per mole of dimethyl terephthalate and preferably from 150 to 500 moles of methanol per mole of dimethyl terephthalate. With the treatment with methanol, up to about 90% of the terephthalic units present in the polyalkylene terephthalate are converted to dimethyl terephthalate in an average treatment time with methanol of 1-10 hours. If desired, the treatment with methanol vapours can be carried out on the polyalkylene terephthalate in solution in one of its organic solvents, or partially degraded. The methanol vapours containing the monomers of polyalkylene terephthalate are continuously removed from the reaction zone and processed to remove the excess methanol and separate the single components. For this purpose, the methanol stream leaving the reactor is subjected to partial condensation and the methanol vapours obtained are further condensed and recycled to phase (a) of the process. The condensate thus obtained is distilled and the vapours at the head comprising alkylene glycol and dialkylene glycol are removed. The products at the bottom comprising dimethyl terephthalate, dihydroxyethyl terephthalate, hydroxyethyl terephthalate and dimethyl isophthalate, are subjected to hydrolysis. The impurities and extraneous materials contained in the starting waste product are periodically discharged from the reactor where they accumulate. The hydrolysis process is carried out with cold or hot water fed to the reactor in such a proportion that the ratio water/product to be hydrolized is higher than 2, preferably between 5 and 20. The process is carried out at a temperature higher than 150°C, preferably at a temperature of between 200 and 300°C for an average residence time of 10-60 minutes. The heating can be carried out by means of an external heating jacket around the reactor or by coils outside or inside the reactor. According to a preferred embodiment, the hydrolysis water is in the form of high pressure vapour fed to the reactor from below. In this way the stirring, thermal exchange and a better contact between the hydrolysis water and material to be treated, are ensured. The resulting solution is subjected to flash at a temperature higher than 100°C, for example between 110 and 150°C, and at a pressure corresponding to the partial pressure of the water at said temperature. The terephthalic acid precipitates and is separated by filtration, using a filter or centrifuge, at the same temperature and pressure. The crystals thus obtained are washed in the same filtration device and dried. The impurities and extraneous materials contained in the starting waste or by-products are discharged with the crystallization mother liquors. The following illustrative but non-limiting examples provide a better understanding of the present invention and refer to a preferred embodiment of the process of the present invention. In the examples, the process of the present invention is described with reference to the diagram of the enclosed figure which represents a schematic indicative view of a preferred plant for its embodiment. In the examples all the parts and percentages are given by weight unless otherwise specified. EXAMPLE The following are continuously fed, under standard conditions, to a reactor (1) having a capacity of 30 l: 300 g/h of waste products of polyethylene terephthalate coming from the processing of bottles and fibres, previously granulated by means of a twin-screw extruder (2) and heated to 260°C, and 8000 g/h of methanol vapours, removed from a tank (3) by means of a pump (4) and superheated to 255°C by a heat exchanger (5). The vapours which developed from the reactor (1), were fed to a condenser (6), maintained at a temperature of 140°C, in which the vapours of dimethyl terephthalate, methylhydroxyethyl terephthalate and ethylene glycol were condensed, whereas the methanol vapours were collected and condensed in a second condenser (7) and then fed to the tank (3) in order to be recycled. After three hours of treatment, the condensed products in the condenser (6) consisted of: 430 g of dimethyl terephthalate, 200 g of methylhydroxyethyl terephthalate and 630 g of ethylene glycol. The condensation products were continuously fed to a distillation column (8), operating under vacuum at a pressure of 5.1 absolute KPa, to separate the ethylene glycol at the head and the other monomers at the bottom. The ethylene glycol was collected in a tank (9) and could be re-used as a monomer in the synthesis of polyethylene ester. The monomers at the bottom were fed at a flow rate of 0.5 l/h to a hydrolysis reactor (10), having a capacity of 2 l and maintained at a temperature of 270°C and a pressure of 8.3 MPa. 3500 g/hr of water removed from the container (11) and preheated to 270°C by an exchanger (12) were continuously and contemporaneously fed to the hydrolysis reactor (10). The monomers at the bottom and water were fed in such a quantity that the average residence time of the reagents in the hydrolysis reactor (10) was 30 minutes. The reaction mass was continuously fed at a flow rate of 4000 g/hr to a flash (13) operating at 128°C and a pressure of about 0.2 MPa. The vapours were recycled after condensation by a condenser (14). Terephthalic acid obtained by hydrolysis precipitated from the solution and the slurry at the bottom was filtered in a centrifuge filter (15); the filtrate was collected in (16) and the aqueous solution of methanol, containing impurities such as isophthalic acid and diethylene glycol, was discharged into (17). The methanol can be recovered from the solution by distillation. 450 g/hr of "fibre grade" terephthalic acid having a purity of 99.9% upon gaschromatographic analysis, were obtained.

Claims: Continuous process for the recovery of terephthalic acid from the waste or used products of polyalkylene terephthalate polymers comprising in subjecting said waste or used products to two consecutive treatments, the first carried out with superheated methanol vapours (methanolysis) and the second with water or water vapour at a high temperature (hydrolysis). Continuous process according to claim 1, comprising in: (a) treating said polymer waste products with methanol vapours, at atmospheric pressure and at a temperature higher than 150°C, to decompose the polymer into its constituent monomers; (b) continuously removing the methanol vapours and monomers obtained in step (a) and entrapped by said methanol vapours; (c) condensing the monomers entrapped by the methanol; (d) separately condensing and recycling the methanol to step (a); (e) removing the glycols from the mixture of condensed monomers by distillation; (f) subjecting the residual product of the distillation to hydrolysis; and (g) recovering the terephthalic acid obtained by crystallization. Continuous process according to claims 1 or 2, wherein the treatment with methanol vapours is carried out at atmospheric pressure, at a temperature higher than 150°C, and with a weight ratio methanol/waste or used materials of at least 2. Continuous process according to claim 3, wherein the weight ratio methanol/waste or used materials is between 3 and 5. Continuous process according to any of the previous claims, wherein the temperature of the methanol vapours is higher than 230°C, and preferably between 240 and 260°C. Continuous process according to any of the previous claims, wherein the methanol vapours leaving the reactor are subjected to partial condensation, the condensate obtained is subjected to distillation and the vapours at the head are removed and the products at the bottom subjected to hydrolysis. Continuous process according to any of the previous claims, wherein the hydrolysis is carried out with cold or hot water with a ratio water/product to be hydrolized higher than 2, at a temperature higher than 150°C and an average residence time of 10-60 minutes. Continuous process according to claim 7, wherein the ratio water/product to be hydrolized is between 5 and 20 and the temperature is between 200 and 300°C. Continuous process according to any of the previous claims, wherein the solution resulting from the hydrolysis process is subjected to flash at a temperature higher than 100°C, preferably between 110 and 150°C, and a pressure corresponding to the partial pressure of the water and said temperature, and the precipitated terephthalic acid is separated by filtration. Continuous process according to any of the previous claims, wherein the polyalkylene terephthalate polymer is polyethylene terephthalate.

Inventors: SOCRATE CONTESSA, VOSA RENATO, SOCRATE, CONTESSA, VOSA, RENATO
Applicants: MONTEFIBRE SPA, MONTEFIBRE S.P.A.
Application year: 1994
Main classification: C07C51
IPC codes: C07C67, C07C51